Existing subscriber loops can readily provide two-way digital transmission (full-duplex) on a pair of wires using analog signals at voice-band frequencies. This is achieved by amplitude-shift keying, phase-shift keying, frequency-shift keying, or other such techniques. However, full-duplex transmission of high-speed digital signals at ultra-sonic bit rates is difficult to achieve on a single communication path. It has been proposed therefore to employ a time compression multiplex (TCM) technique on a half-duplex transmission system wherein a burst-mode or ping-pong approach is utilized.
Typically in such TCM systems, the digital information signal to be transmitted is divided into discrete portions and each portion compressed with respect to time to form a so-called "burst", occupying less than one half the time of the original portion. The transmitter at each terminal alternately transmits the burst onto the path, following which the associated receiver at each terminal can receive a corresponding burst from the other transmitter. On receipt, each burst is expanded to occupy its original time span. Externally, the system appears to be transmitting the two digital information streams continuously and simultaneously i.e. full-duplex communication. So far as the transmission path is concerned, however, half-duplex transmission takes place with alternate bursts travelling in opposite directions.
Having transmitted its own burst, each transmitter must wait until the incoming burst from the other transmitter has been cleared from the communication path before it can transmit again. Arrival of the incoming burst will be delayed by at least a time interval equal to twice the transmission delay or propagation time of the path. The time interval (dead time) detracts from the efficiency of utilization of the communication path. Thus, for a given burst length, the efficiency decreases as the path length increases. The efficiency can be improved, for a given path length, by increasing the length of each burst, thus increasing the "on" time relative to the "dead" time. However, this exacerbates the synchronizing timing problem by increasing the corresponding reception interval during which the receiver is turned off and hence the receiver's clock receives no control bits to keep it synchronized.
Each receiver must be synchronized to the other's transmitter. U.S. Pat. No. 4,049,908, issued Sept. 20, 1977 and entitled "Method and Apparatus for Digital Data Transmission" describes a system in which a single pulse is transmitted at the beginning of each burst to establish synchronization. A paper entitled "A Long Burst Time-Shared Digital Transmission System for Subscriber Loops" by J. P. Andry et al, Societe Anonyme de Telecommunications, Paris, France, International Symposium on Subscriber Loops and Services 80, pp 31-35; describes an alternate system in which two synchronization framing bits are transmitted at the beginning of each burst.
Such systems function well on short loops, particularly with short bursts, in which strong signals are received. However, on long loops spurious signals resulting from cable irregularities such as gauge changes and bridged taps (which cause reflected pulses), can cause false synchronization to be established. This problem can be alleviated by providing a guard time (as described in U.S. Pat. No. 4,049,908) or by adding a unique sequence of much longer synchronization bits at the commencement of each burst. However, both of these solutions further reduce the data transmission efficiency. Consequently, a problem arises in establishing and maintaining frame synchronization and bit timing between the two terminals utilizing a minimum number of bits.
In a paper by R. Montemurro et al entitled "Realisation d'un equipement terminal numerique d'abonne pour service telephonique et de donnees", colloque international de commutation, International Switching Symposium, Paris, May 11, 1979, pp 926-933; there is described a synchronization technique in which two frame bits are added, one at the beginning and the other at the end of each burst. With this arrangement, false synchronization is more readily prevented than in the other systems since it can only occur if one or the other of the bits which was erroneously detected as a true synchronization bit, is outside the burst. Thus, essentially the only condition that can cause false synchronization to be detected is one in which the two detected bits, one a spurious bit and the other a signal bit, have the correct polarity and are spaced from one another by the correct interval. However, such a system still utilizes a guard time to insure that adequate decay of all reflected signals takes place before signal transmission commences in the opposite direction.